A. Field of the Invention
This invention relates generally to optical instruments, and more particularly, to a device usable for reducing undesired background light resulting from sources of background light that are both internal and external to the optical instruments.
Whenever an object is observed through an optical device, such as a telescope or a microscope, or the like, the image of the object is generally surrounded by a field of unwanted background radiation that occurs as a result of sources internal and external to the optical device. The internal sources of unwanted background radiation include reflection between the optical elements, diffraction by the effective aperture stop of the optical instrument (for a telescope, it is usually the clear aperture of the objective), lens irregularities, such as bubbles and other inclusions, dust and scratches on the lens surfaces, and thermal radiation in infrared work. The major external source of scattered light is the atmosphere, dust and other particles suspended within the atmosphere, and thermal radiation in the infrared.
While in many instances the magnitude of the unwanted background radiation is low relative to the brightness of the image and does not adversely affect the quality of the image, in other instances where the brightness of the desired image is quite low, the background radiation can obscure the image of the object being viewed. Such a problem is particularly acute in astronomic telescopes, particularly when such telescopes are being used either near light generating urban areas, for viewing objects near the periphery of the sun, or for viewing stars in the infrared.
B. Description of the Prior Art
Several methods for reducing internally generated background radiation are known. These include the use of coated optical surfaces to reduce surface reflectivity, the use of a field lens to image the objective lens onto an aperture stop to block out the diffraction ring of the objective lens, the use of an occulting disc within the telescope to eliminate background light produced by multiple reflections and the use of an external occulting disc to block the direct light of the sun before it reaches the instrument.
While these techniques substantially reduce the amount of background radiation produced by the instrument itself, the implementation of such techniques requires extensive modification of the instrument. Moreover, such techniques require the use of apparatus that is costly, cumbersome and requires a high degree of maintenance to assure that the lenses are always kept clean. More importantly, these techniques are not able to affect any reduction in the externally generated background radiation.
Presently, no instruments exist for eliminating externally scattered light optically. The presently known schemes for eliminating externally scattered light utilize techniques where the intensity of the background light at a portion of the field adjacent to the image is sampled and subsequently subtracted from the intensity of the field containing the image of the object under investigation. After a sufficiently long integration time, a weak signal, such as a signal representative of the image of a star, may be pulled from the "noise" resulting from the background light intensity. This image may then be displayed on a cathode ray tube or other display device for viewing.
There are several properties of the above scheme to reduce the effects of background light that should be noted. It is effective only when (1) the background light in one region of the field does not fluctuate rapidly in time with respect to that in an adjacent region and (2) the scale of the spatial structure of the background light in the field is much larger than the scale of the image of the desired object. This poses serious limitations on the value of this technique. There is a further constraint on the utility of such a scheme: since the optical detectors used with such circuitry detect the impact of photons on a photoelectric surface, rather than detecting an incident wave, the amount of reduction in the background noise is limited by the shot noise resulting from the photon detection process. These boundaries represent serious restrictions on the applicability of this technique. Lastly, when such a scheme is applicable, it generally requires complex electronic circuitry, such as a computer to achieve the desired result.